Support pushing single files in PushReleaseArtifacts()
Support pushing single files in PushReleaseArtifacts()

This commit modifies the function Instance.PushReleaseArtifacts() in
the build package to support pushing single files to the release buckets.

Before, PushReleaseArtifacts() would fail if applied to a single file.

Signed-off-by: Adolfo García Veytia (Puerco) <9de42c8261a7cacaae239656a4dd6991d9b9250f@uservers.net>

kubernetes/release,kubernetes/release